A wire electrode with a jacket having a low volatilization temperature, is already known from DE-P 29 06 245 (corresponds to U.S. Pat. No. 4 287 404). The jacket of the eroding wire has the advantage that a high-tensile eroding wire can be used, which during erosion can be maintained under a high tension by means of tensioning devices so that a clean cut is created. A strand breakage during cutting can now be avoided since the metal layer of the jacket forms a thermal shield, which furthermore,because of its small thickness, is quickly pierced by the discharge crater and releases alloy parts of a low volatilization temperature, as for example zinc. In this respect a covered wire electrode is substantially more advantageous than a not covered one.
It is furthermore known from Japanese Offenlegungsschrift 50-102999, cited in DE-P 29 06 245, Column 2, lines 20 to 24, to cover a high-tensile material wire electrode with a highly conductive metal, such as gold or silver, in order to protect the electrode against loss of its flexibility due to contamination during discharge. An improvement of the cutting characteristics of such a wire electrode could not be found (see the reference). Rather the cutting characteristics correspond only with those which the electrode wire itself has. Such wire electrodes with silver coatings have therefore not been successful but instead those with jackets of metals or metal alloys, which have at least a low-melting part, have.
It is desired in every case that the cutting performance, namely, the removal performance in mm.sup.2 /min and/or the surface quality is further improved.
It is therefore the purpose of the invention to provide a wire electrode of the type identified in detail above, in which without any significant extra expense the removal performance and the finished surface quality is still further increased.
The purpose is attained according to the invention by the wire electrode being covered with a surface layer of an electrically highly conductive material, such as silver, gold, rhodium, palladium, iridium or a very thin surface layer thickness of their alloys. It is advantageous for economical reasons that the surface layer consists of silver. It has proven to be particularly advantageous when the surface layer has a layer thickness of 0.05 to 0.8 .mu.m. Tests have shown that already an effect worth mentioning results starting with layer thicknesses of 0.02 .mu.m. It is advantageous when the core of the eroding wire consists of an electrically well conducting material, such as copper, brass, (Cu-Zn), bronze, (Cu-Sn), steel with a Cu-jacket, tungsten, molybdenum or the like.
Already in the above-mentioned DE-P 29 06 245 the thought was considered to surround the eroding wire first with a copper or silver layer, which is then covered with the metal layer effecting the thermal protection. An increase in the cutting performance, however, was not achieved in this manner. The invention moves away from both directions described in detail above: On the one hand, the highly conductive layer is now not applied between eroding wire and jacket but to the outside of the jacket, if a coated eroding wire is used at all. On the other hand, it is kept extremely thin and especially the extremely thin layer thickness results in an increase in the cutting performance of approximately 20%. The evenly thin silver layer, which reduces the wear of the electrode and has a high conductivity, increases the current density. This extremely thin, highly conductive, layer does thereby also not hinder the volatilization of the low-volatilizing alloy parts lying below the layer. It is conceivable, as an explanation of this phenomena, that it is the result of the skin effect .